1. Field of the Invention
The present invention relates to silver halide photographic emulsions which are spectrally sensitized and, more particularly, to silver halide photographic emulsions containing merocyanine dyes which have improved spectral sensitization.
2. Description of the Prior Art
Various classes of dyes are known to be effective for use with the intention of spectrally sensitizing silver halide photographic emulsions. Of such classes, cyanine dyes and merocyanine dyes are predominantly employed. In particular, when the combination of two or more dyes, or that of a certain sensitizing dye and a certain compound which itself exhibits no sensitizing action or an extremely poor sensitizing action can exhibit a greater sensitization than the sum of the sensitivity achieved using each compound separately, such an action is known as supersensitization.
Various compounds which give rise to supersensitization when used in combination with merocyanine dyes are known. For instance, examples of such are disclosed in U.S. Pat. Nos. 3,480,434, 3,672,897, 3,703,377 and so on. However, these dyes give rise to a supersensitization in an efficient manner only with merocyanine dyes and the spectral sensitivity is restricted and limited to a particular wavelength region. Accordingly, the use of those dyes described above is limited to special cases. Therefore, discovery of supersensitization techniques which are applicable to combinations of a wide variety of merocyanine dyes and emulsions have been desired.
Thus far benzotriazole compounds are known as an anti-foggant, and are used with the intention of reducing fog. Moreover, it is well known that benzotriazole compounds frequently decrease photographic sensitivity. (For example, such desensitization action is described in Thomas R. Tomson, "Action of Organic Stabilizers on a Photographic Emulsion", Photographic Science and Engineering, Vol. 3, page 272, (1959) and in M.R.V. Sahyun, "Interaction of Benzotriazole with Development and Fog Centers", Photographic Science and Engineering, Vol. 15, page 48, (1971).) On the other hand, recently in Japanese Patent Application 7309/75 (corresponding to U.S. Patent Application Ser. No. 649,000, filed on Jan. 14, 1976) the fact that the photographic sensitivity of silver halide emulsions containing carbocyanine or dicarbocyanine dyes was markedly increased by the addition of halogenated benzotriazole compounds was disclosed.
Contrary to expectations, it has now been found out that the efficiency of spectral sensitization in silver halide emulsions containing, in particular, merocyanine dyes, free of acidic groups, is remarkably increased by the addition of benzotriazole compounds.